It is common for an arc discharge lamp to have an electrode with a massive head formed on the interior end of a rod. For example, many metal halide high-intensity discharge lamps use an electrode with a straight tungsten rod wrapped with a coil to form the head. During operation the wrapped head provides a larger area from which thermionic electrons are emitted, resulting in a more durable electrode that operates at lower temperatures. Unfortunately, the massive head is difficult to heat initially and lamp starting may suffer. If the wrapped head is too large, a high temperature spot mode arc attachment can occur that degrades the steady-state operation of the lamp, especially when no emitter material is used. Coil wrapped electrodes can also have large performance variabilities, likely due to the variable heat connection between the rod and coil. All of these effects can result in excessive electrode evaporation and sputtering. The evaporated electrode material then blackens the arc tube walls. There is then a need for an electrode with good starting features and good heat control.
One method to improve starting and lower the temperature of the electrode head is to include thoria in the electrode. Use of thoriated electrodes in metal-halide, high-intensity discharge (HID) lamps can result in excellent color and high-efficacy in a small volume with an electrode lifetime of 8,000 to 20,000 hours. Typically, this long lifetime or high-maintenance is achieved by doping the electrodes with thoria emitter to reduce the work function of the electrode and therefore lower the electrode temperature. However, thoria is felt to be environmentally undesirable. Removal of thoria is especially difficult in general lighting applications using metal-halide lamps where the electrode must function well for starting and during steady-state alternating-current (AC) operation and the resulting evaporation. There is then a need for a thoria free electrode with good starting and with good steady-state characteristics
The most common approach to achieve good lifetime with a non-thoriated electrode is to use the conventional coiled electrode configuration, but without the use of emitter materials. Such an electrode consists of a tungsten rod with a tungsten coil wrapped around the rod, usually near the tip. In the cathode phase, the additional surface area of the coil provides additional arc attachment area, provided the electrode operates in a diffuse attachment mode. This lowers the tip temperature because less thermionic emission is needed to supply the needed current. In the anode phase, the tip temperature is determined primarily by the balance of heat input from recombination of hot plasma electrons with bulk metal of the electrode and the radiation and conductive losses down the electrode stem. During the first few seconds of the starting phase, the coil also provides an attachment region for the glow phase and subsequent thermionic phase. Thoria free electrodes have been shown to give reasonable performance when rare-earth/alkali metal halide fills are used, particularly with ceramic arc tubes. This appears to be the result of the rare earth or alkali vapor functioning as an emitter material. However, an electrode that has a relatively low electrode tip temperature without thoria emitters for a broad range of metal halide fills and lamp types is highly desirable.
The coil and rod approach to a thoria-free electrode has a number of disadvantages however. The most significant is that coil-rod system is not well suited to large tip areas. First, the poor thermal interfaces between coil windings and the coil and rod cannot transfer heat efficiently, particularly when the components are large. The interfaces can then induce regions of localized heating. The increased thermionic emission from the hotter regions increases the local heat flux and can result in undesirable spot arc attachment. This mode of operation has very high, localized temperatures for tungsten electrodes without emitters, and leads to excessive evaporation of electrode material, and flickering of the arc
The second problem with large coils is slow starting. The power deposition into the massive coil and rod is not large enough to rapidly raise the tip temperature to high enough values for good thermionic emission. The massive electrode coil can let the discharge linger in the glow stage. This is particularly troublesome without an emitter to reduce the glow-to-arc transition temperature. U.S. Pat. No. 6,614,187, describes a short arc mercury lamp with a coil configuration with good contact to the rod while a second part of the coil does not contact the rod. This improves the glow-to-arc transition and transfer of thermionic emission to the rod during starting. However, the coil construction is complicated, requiring steps to sinter or melt tungsten powder between rod and coil and special coil winding steps to produce a graded coil diameter.
Other approaches to thoria-free electrodes have been disclosed which use alternative non-radioactive emitter materials. U.S. Pat. No. 5,712,531 Rademacher, describes the use of a lanthanum oxide emitter in a 2000-Watt metal-halide lamp. This emitter material is not chemically stable with many light-emitting metal-halide fills and evaporates much more rapidly than thoria, thus having limited use for long-life general lighting applications. The emitter is also supplied as a pellet that must be enclosed in an electrode coil, adding to cost and complexity. U.S. Pat. No. 3,916,241 Pollard, describes the use of a recess in the tip to form a dispenser of emitter material for a mercury arc lamp. The use of non-thoriated emitters have the same disadvantages as Rademacher in metal-halide discharge lamps and the recess is used only to protect the emitter from direct contact by the discharge stream. U.S. Pat. No. 6,046,544 Daemen, discloses a three-component emitter in which the emitter material is supplied as a sintered electrode or as a pellet. As stated in Daemen, the sintered form is not useful in many applications because of depletion by evaporation. The pellet form also requires additional structure to support it.
Approaches to non-radioactive electrodes based on different electrode structures without any additional emitter materials are disclosed in WO 01/86693 Theodorus; EP 1 056 115 Yoshiharu; WO 03/060974 Haacke; and U.S. Pat. No. 6,437,509 Eggers. Theodorus discloses the use of emitter-free tungsten materials in which a second tungsten filament coil is completely enclosed by the primary tip coil to aid starting without the use of emitter materials. The configuration reduces tungsten sputtering because of the enclosing space of the primary coil. While this configuration improves starting maintenance, the manufacturing complexity and basic issues associated with a coil at the tip are not resolved.
The Yoshiharu patent describes an improvement to the standard rod and coil electrode by replacing the coil with a solid emitter-free tungsten cylinder that is welded to the rod. This overcomes many of the problems associated with the coil at the tip. The electrode in Yosiharu cannot reach the large optimal tip area because heating such a large electrode mass during the starting phase causes a long glow-to-arc transition over a large electrode surface area. This results in excessive tungsten sputtering that blackens the lamp. Haacke discloses a similar electrode having a large solid head for automotive discharge lamps. In this design, the head is partially fused to the quartz arc tube. The design prevents overheating during the high-current instant-light requirement for automotive applications, but is not readily adaptable to higher-wattage general lighting situations where the glow-to-arc transition would be difficult. Additionally, automotive HID lamps operate at very high pressures that reduce wall blackening and have lower life requirements than general lighting HID lamps. Eggers discloses configurations in which the use of single or multiple solid cooling bodies surround a tungsten rod and are laser-welded to the rod. However, unless special lamp and electrode conditions are met, the structure in Eggers has similar starting difficulties under conditions when tip area is large. A cooling structure similar to Egger's is also disclosed in U.S. Pat. No. 6,211,615 Altmann, but again without mention of special lamp and electrode conditions needed to improve starting. Furthermore, all of these disclosures do not disclose the special electrode, lamp, and ballast conditions necessary for achieving improved steady-state maintenance without spot attachment.
Accordingly, there exists a need for an electrode that provides improved steady-state maintenance by increasing the tip area without spot attachment while simultaneously having good starting maintenance. This is particularly true for higher current electrodes. Additionally, optimal performance electrodes should have the advantages of reduced manufacturing variability and have simple structures for optimization by computer simulation. There is a need for an electrode with good life, and maintenance in a dimming operation mode.